1. Field of the Invention
The invention relates to the preparation of solid-state semiconductor photonic devices such as optoelectronic devices and, more particularly, to the preparation and utilization of solid-state semiconductor photonic devices for producing electricity from metal-semiconductor (Schottky-barrier), metal-insulator-semiconductor, semiconductor-semiconductor and semiconductor-insulator-semiconductor heterojunction photovoltaic cells for converting electricity to optical radiation, and for detecting or measuring optical signals through electronic processes.
2. Description of the Prior Art
Photonic devices typically involve electrical or electronic devices in which photons play a major role. Solid-state semiconductor photonic devices are generally divided into three groups. In one group, photodetectors measure or detect optical signals through electronic processes. Examples of the photodetector group include photoconductors, photodiodes, avalanche diodes and phototransistors. In a second group, devices convert electrial energy into optical radiation and include light-emitting diodes (LEDs) and diode lasers. The third group relates to devices converting optical radiation to electrical energy and includes solid-state heterojunction photovoltaic cells. These photonic devices utilize at least two layers of materially different solids (i.e. solid-state heterojunction), one of which is a semiconductor, to convert energy from light into electrical energy or electrical energy into light (Note: The two layers of solids may also be of the same semiconductor material and form a solid-state homojunction; however, in this case the two layers of semiconductor contain oppositely doped resistivity types, i.e. n-p or p-n. As used herein, reference to heterojunction also includes the homojunction of two semiconductor layers of appropriately doped resistivities.)
The solid-state heterojunction photovoltaic cell has recently received considerable attention. Such a cell utilizes at least two layers of materially different solids, one of which is generally a semiconductor to convert energy from light (usually solar energy) into electrical energy. These solid materials, according to the band theory of solids, contain atoms whose discrete electronic energy states have merged into energy bands of allowed energies for electrons. The energy required to excite electrons in such solid materials from a maximum energy in the valence band to a minimum energy in the conduction band represents the band gap energy. At approximately room temperature, valence and conduction energy bands of conductors such as metallic solids are not separated, i.e. they have a band gap of about 0. Semiconductor solid materials, on the other hand, are typically separated by a band gap of above 0 to less than about 4.0 e.V., while higher values are associated with insulator materials.
The most efficient utilization of terrestrial solar energy by semiconductor materials has been observed to occur with the absorption of photons associated with near-infrared light. Light-absorbing semiconductor materials having a band gap of approximately 1.4 e.V. tend to maximize the efficiencies of the conversion from solar to other forms of energy. In photovoltaic cells, electron-hole pairs are generated by the absorption of light in semiconductors. The electron and the hole of electron-hole pairs are separated at a metal-semiconductor (M-S) junction, a metal-insulator-semiconductor (M-I-S) junction, a semiconductor-semiconductor (S-S) junction, or at the junction of two semiconductors having a thin layer of insulator material between them (S-I-S), and are injected at respective sides of the junction to produce electrical energy. Holes and electrons move to the surface or bulk of the semiconductor, depending on their resistivity category, i.e. n-type or p-type.
One of the problems with solid-state semiconductor photonic devices is efficiency. A number of approaches have been taken to increase the efficiency of such devices. In one approach, a cell with an M-I-S or S-I-S junction includes a coating material, such as a highly conductive metal like platinum, or a semiconductor having a wide band gap (i.e., &gt;3.0 e.V.), such as In.sub.2 O.sub.3 or SnO.sub.2, on the surface of the solid-state device. However, the search continues for coating materials that impart higher efficiencies to semiconductors useful in solid-state semiconductor photonic devices especially in heterojunction photovoltaic cells. Furthermore, the search continues for more effective methods of preparing such devices.
Accordingly, it is an object of the present invention to provide a solid-state semiconductor photonic device, and more particularly a heterojunction photovoltaic device.
Another object of the invention is to provide a method for preparing a solid-state semiconductor photonic device containing a coating material on a semiconductor.
Yet another object still is to provide a method for producing a solid-state photovoltaic device exhibiting highly efficient photocurrent generation in a photovoltaic cell.
A further object is to provide a method for depositing a material onto a semiconductor by photo-assisted techniques.
These and other objects and advantages of the invention will become apparent from the following description